Chemical process

ABSTRACT

1,1,1,2-tetrafluoroethane (HFA 134a) is manufactured from trichloroethylene by a two-stage process comprising reacting trichloroethylene with hydrogen fluoride under superatmospheric pressure in a first reaction zone to form 1,1,1-trifluoro-2-chloroethane (133a) and reacting the 1,1,1-trifluoro-2-chloroethane with hydrogen fluoride in a second reaction zone to form 1,1,1,2-tetrafluoroethane; the entire product stream from the 133a reaction zone together with additional HF of required is fed through the 134a reaction zone. The two reaction zones may be provided within a single reaction vessel.

This is a continuation of application No. 07/676,703 filed on Mar. 29,1991, which was abandoned upon the filing hereof.

This invention relates to a chemical process and more particularly to aprocess for the manufacture of 1,1,1,2-tetrafluoroethane, knowngenerally as HFA 134a.

Several methods have been proposed for the manufacture of1,1,1,2-tetrafluoroethane (HFA 134a) which is useful as a replacementfor CFCs in refrigeration and other applications. In United KingdomPatent Specification No. 1,589,924 there is described the production ofHFA 134a by the vapour phase fluorination of1,1,1-trifluoro-2-chloroethane (HCFC 133a) which is itself obtainable bythe fluorination of trichloroethylene as described in United KingdomPatent Specification No. 1,307,224.

The formation of HFA 134a as a minor product of the fluorination oftrichloroethylene is described in United Kingdom Patent SpecificationNo. 819,849, the major reaction product being HCFC 133a. In WO 90/08755there is described the conversion of trichloroethylene to HFA 134awherein the two-stage reactions are carried out in a single reactionzone with recycle of part of the product stream.

Carrying out the conversion in a single reaction zone as described in WO90/08755 suffers from the serious drawback that the fluorinationcatalyst tends to deactivate rapidly, largely as a result of carbondeposition and thus has a very short lifetime. For example we have foundthat operation of the single zone process using a chromia catalyst at340° C. with a feed containing 10 molar % trichloroethylene and acontact time of 20 seconds resulted in fall in conversion of organics inthe feed stream to HFA 134a to below 10% in a matter of less than 24hours and that in order to maintain a conversion of 10% it was necessaryto raise the temperature of the catalyst by some 30° to 40° C. (from340° to 370°-380° C.) over a period of 4 days. This problem of shortcatalyst lifetime renders the single stage process unsuitable ofpractical adoption.

It has now been found that a two-step reaction sequence carried out inseparate reaction zones as hereinafter described provides significantlyimproved yields of the desired product with high catalyst selectivityand high catalyst productivity and with an increased catalyst lifetime.For example, operation of the process of the invention under theconditions described above but with a 10 secs contact time in eachreaction zone resulted in insignificant ageing of the catalyst over aperiod of 4 days; a rise in temperature by about 2° C. over the periodof 4 days was sufficient to maintain a conversion of organics in excessof 10%.

It has been found that carrying out the two-step conversion oftrichloroethylene to HFA 134a in separate reaction zones of equal sizeat atmospheric pressure in each reaction zone is impractical in thatonly low conversions of trichloroethylene, for example 20-30%, areobtained in the first reaction zone. The present invention enablestrichlorethylene conversions of 90% or greater to be achieved.

According to the invention, there is provided a method for themanufacture of 1,1,1,2-tetrafluoroethane which comprises the steps of:

(A) contacting a mixture of trichloroethylene and hydrogen fluoride witha fluorination catalyst under super atmospheric pressure at atemperature in the range of about 200° to 400° C. in a first reactionzone to form a product containing 1,1,1-trifluoro-2-chloroethane andhydrogen chloride together with unreacted starting materials,

(B) passing the total product of step A together with hydrogen fluorideto a second reaction zone containing a fluorination catalyst at atemperature in the range of about 280°-450° C. but higher than thetemperature in step A to form a product containing1,1,1,2-tetrafluoroethane, 1,1,1-trifluoro-2-chloroethane and hydrogenchloride,

(C) treating the product of step B to separate 1,1,1,2-tetrafluoroethaneand hydrogen chloride from 1,1,1-trifluoro-2-chloroethane and unreactedhydrogen fluoride,

(D) feeding the 1,1,1-trifluoro-2-chloroethane mixture obtained fromstep C together with trichloroethylene and hydrogen fluoride to saidfirst reaction zone (step A), and

(E) recovering 1,1,1,2-tetrafluoroethane from the1,1,1,2-tetrafluoroethane and hydrogen chloride separated out in step C.

The fluorination catalysts employed in steps A and B of the method ofthe invention may be the same or different (though preferably are thesame) and may be supported or unsupported. Any of the fluorinationcatalysts described in the prior art may be used including variousinorganic compounds, for example oxides, halides and oxyhalides ofmetals such as aluminium, cobalt, manganese, iron and especiallychromium. Suitable chromium-containing catalysts include the oxide,hydroxide, oxyhalide, halides, inorganic acid salts, basic chromiumfluoride and the catalysts described in United Kingdom PatentSpecification No. 1,307,224. Preferred catalysts are chromia and a zincor nickel promoted chromia. Such catalysts may be given aprefluorination treatment by passing hydrogen fluoride with or withoutnitrogen diluent over the catalyst at about 250°-450° C. to conditionthe catalyst prior to use.

The catalysts may be compressed into pellets and used in a fixed bed or,alternatively, catalysts of appropriate particle size may be used in amoving bed such as a fluidised bed.

A wide range of amounts of hydrogen fluoride may be employed in step Bof the method of the invention, ranging from well below thestoichiometric amount to well above this amount. Typical amounts includefrom 1 to 10 moles, and preferably from 2 to 6 moles, of hydrogenfluoride per mole of 1,1,1-trifluoro-2-chloroethane. Accordingly, theproduct of this reaction step will usually contain unreacted hydrogenfluoride in addition to 1,1,1,2-tetrafluoroethane, hydrogen chloride andby-products. Preferred reaction temperatures for this stage of theprocess are in the range from 285° to 385° C. especially 300° to 385° C.and more especially 325° to 385° C., with contact times of from 1 to 100and preferably from 5 to 30 seconds at a pressure of 5 to 20 bars.

From 10 to 100, preferably from 15 to 60, moles of hydrogen fluoride permole of trichloroethylene are typically employed in Step A. Again, thereaction product of this stage will normally contain unreacted hydrogenfluoride and perhaps low levels of unreacted trichloroethylene. Contacttimes of up to 100 seconds, preferably 5 to 30 seconds may be used,typically at 220°-350° C. and 5 to 20 bars pressure.

Step A is carried out under the superatmospheric pressure which ispreferably at least 2 bars and more preferably at least 5 bars. Ingeneral, increasing the pressure results in an increase in catalystproductivity in step A. In practice the pressure will usually not exceed30 bars. Step B may be carried out at atmospheric or superatmosphericpressure but in practice the pressure in step B will usually be the sameas that in step A. In addition step C will usually be carried out atapproximately the same pressure as steps A and B.

The reaction and separation steps which make up the method of theinvention may be performed using conventional equipment and techniques.Thus, for example, recovery of 1,1,1,2-tetrafluoroethane in step E maybe effected by washing the gaseous tetrafluoroethane with water andaqueous sodium hydroxide solution and then drying and condensing thetetrafluoroethane.

It is preferred that the method of the invention is operatedcontinuously. In practice, however, catalyst deactivation usually occursrequiring discontinuous operation of the process to permit catalystregeneration or reactivation which may be conveniently effected bypassing air or a mixture of air and inert gas, for example nitrogen,over the catalyst at a temperature in the range of 300° to 500° C. Apreferred catalyst reactivation process comprises heating the catalystin a mixture of air and hydrogen fluoride, the resulting hot hydrogenfluoride being useable directly in step A and/or step B of the methodaccording to the invention. The frequency of catalyst regeneration maybe reduced if air is added to the reaction mixture in step A and step Bof the process.

A particularly useful feature of the invention is that the exothermicconversion of trichloroethylene to 1,1,1-trifluoro-2-chloroethane instep A may be performed in a low cost adiabatic reactor, therebyproviding significant cost advantages over reactor systems employinginternal cooling surfaces. If desired, step B may also be carried out inan adiabatic reactor, using an interstage heater to raise thetemperature of the gas stream between the two reactors.

The temperature employed in step A of the process is lower than thetemperature employed in step B of the process. The recycle stream fromstep B may require cooling to or to below the temperature used in step Aand a useful technique comprises mixing the trichloroethylene feed tostep A with the recycle stream in advance of the step A reactor; in thisway the recycle stream is cooled by the trichloroethylene whilst at thesame time the trichloroethylene is heated, thereby reducing the need forexternal heating.

Separation of 1,1,1,2-tetrafluoroethane and hydrogen chloride from theproduct stream in step C of the process may be effected in anyconvenient manner, for example using a distillation technique.

As stated, the 1,1,1,2-tetrafluoroethane production process is carriedout in two reaction zones operated at different temperatures. The tworeaction zones may be provided in separate reactors if desired, but in apreferred feature of the invention the process is carried out in asingle reactor containing both of the reaction zones. Thus, for example,the reactor may comprise a series of tubes through which the reactantsteams are fed, each tube containing the fluorination catalyst andhaving a lower temperature length (for step A) and a higher temperaturelength for (step B). Trichloroethylene and hydrogen fluoride, togetherwith a recycle stream (step D) are fed into the lower temperature end ofthe tube and a product stream containing 1,1,1,2-tetrafluoroethane iswithdrawn from the higher temperature end of the tube. The reactionvessel may be an adiabatic reactor.

HFA 134a produced by the process of the invention contains a smallamount, for example 200 to 1000 ppm, of the toxic impurity1-chloro-2,2-difluoroethylene, commonly known as 1122. The procedureemployed in the work-up of the product stream from step B (including theseparation of step C) will usually contain one or more provisions forremoving the 1122 which owing to its similar boiling point to HFA 134atends to stay with the HFA 134a during the work-up operations.

At least part of the 1122 can be removed from the product stream priorto separation step C by contacting the product stream from step Btogether with hydrogen fluoride (already present in the product stream)over a fluorination catalyst such as chromia at a temperature in therange of about 150° to 250° C.

In the preferred embodiment of the invention described above in whichstep A and step B are carried out in different reaction zones of asingle reactor, there may be provided a third reaction zone for treatingthe HFA 134a product stream with HF at a low temperature to remove atleast part of the 1122 present in that product stream. Thus, forexample, in the tube reactor described above, each tube containing afluorination catalyst such as chromia may comprise a first zonemaintained at a first temperature for carrying out step A, a second zonemaintained at a higher temperature for carrying out step B and a thirdzone maintained at a lower temperature for carrying out 1122 removalfrom the product stream. Sufficient HF is fed with trichloroethylene(and the recycle stream) into the end of the tube to carry through tothe second and third reaction zones.

Any 1122 present in the HFA 134a after step C can be removed byazeotropic distillation or extractive distillation and/or by contactingthe HFA 134a with a zeolite molecular sieve.

The invention is illustrated but not limited by the following Example.

EXAMPLE 1

1,1,1,2-tetrafluoroethane was produced in a two-reactor systemcomprising a first reactor for converting trichloroethylene to1,1,1-trifluoro-2-chloroethane (step A) and a second reactor forconverting the 1,1,1-trifluoro-2-chloroethane from step A to1,1,1,2-tetrafluoroethane (step B). Trichloroethylene and hydrogenfluoride were fed to the first, low temperature reactor (273° C.) at13.5 bar. g. to convert the trichloroethylene selectively to1,1,1-trifluoro-2-chloroethane (133a). The products of reactor 1 werethen passed to a second, higher temperature, reactor operating at 366°C. and 13.5 bar. g. where the 133a produced in the first reactor waspartially converted to HFA 134a. 133a was included in the feed to the1st reactor together with the hydrogen fluoride and trichloro-ethyleneto simulate a typical feed including recycle of 133a, HF and a smallamount of trichloroethylene from the second reactor. Using anHF:Organics molar ratio of 3.5:1 at the first stage, and a 15% molartrichloroethylene content in the organics feed 133 a to give a contacttime of 13.5 seconds in each reactor, the reaction efficiencies for thetwo reactor system were measured and these are presented in Table 1.

For purposes of comparison, the above procedure was repeated using thesame reactors but carrying out the reactions in both reactors atatmospheric pressure (contact time approximately 1 second). Reactionefficiencies are shown in Table 1. The results in Table 1 show the muchimproved catalyst productivity achieved by carrying out the reaction inthe first reactor at superatmospheric pressure.

The process according to the invention was found to give significantcatalyst productivity advantages as well as high reaction selectivity.

                                      TABLE 1                                     __________________________________________________________________________                Trichloro-                                                                    ethylene                                                                            % Yields from  R134a R134a + R133a                          Reactor                                                                              Reactor                                                                            Conversion                                                                          trichloroethylene                                                                            Selectivity                                                                         Selectivity                            No. 1  No. 2                                                                              (%)   R133a                                                                             R134a                                                                             By-Products                                                                          (%)   (%)                                    __________________________________________________________________________    Invention                                                                            366° C.                                                                     99.5  16.7                                                                              76.3                                                                               6.5   76.6  91.4                                   (super-                                                                       atmospheric                                                                   pressure)                                                                     273° C.                                                                (after 24 hrs)                                                                Comparison                                                                           366° C.                                                                     97.0  59.8                                                                              26.2                                                                              11.0   27.01 88.7                                   (atmospheric                                                                  pressure)                                                                     273° C.                                                                (after 24 hrs)                                                                __________________________________________________________________________

We claim:
 1. A method for the manufacture of 1,1,1,2-tetrafluoroethanewhich comprises the steps of:(A) contacting a mixture oftrichloroethylene and hydrogen fluoride with a fluorination catalystunder superatmospheric pressure at a temperature in the range of about200° to 400° C. in a first reaction zone to form a product containing1,1,1-trifluoro-2-chloroethane and hydrogen chloride together withunreacted starting materials, (B) passing product of step (A) togetherwith hydrogen fluoride to a second reaction zone containing afluorination catalyst at a temperature in the range of about 280°-450°C. but higher than the temperature in step (A) to form a productcontaining 1,1,1,2-tetrafluoroethane, 1,1,1-trifluoro-2-chloroethane andhydrogen chloride, (C) treating the product of step (B) to separate1,1,1,2-tetrafluoroethane and hydrogen chloride from1,1,1-trifluoro-2-chloroethane and unreacted hydrogen fluoride, (D)feeding the 1,1,1-trifluoro-2-chloroethane mixture obtained from step(C) together with trichloroethylene and hydrogen fluoride to said firstreaction zone (step (A)), and (E) recovering the1,1,1,2-tetrafluoroethane and hydrogen chloride separated out in step(C).
 2. A method as claimed in claim 1, wherein 15 to 60 moles ofhydrogen fluoride per mole of trichloroethylene are fed into the firstreaction zone in step (A).
 3. A method as claimed in claim 1 or claim 2wherein the temperature in the first reaction zone in step A is in therange of from 220° C. to 350° C.
 4. A method as claimed in claim 1wherein 2 to 6 moles of hydrogen fluoride per mole of1,1,1-trifluoro-2-chloroethane are fed into the second reaction zone instep (B).
 5. A method as claimed in claim 1 wherein the temperature inthe second reaction zone in step (B) is in the range of from 305° C. to385° C.
 6. A method as claimed in claim 1 wherein the contact time instep (A) and in step (B) is from 5 seconds to 30 seconds.
 7. A method asclaimed in claim 1 wherein the reactions in step (A) and step (B) arecarried out at a pressure of from 5 bars to 20 bars.
 8. A method asclaimed in claim 1 which is operated continuously.
 9. A method asclaimed in claim 1 wherein said first and second reaction zones areprovided by adiabatic reactors.
 10. A method as claimed in claim 1wherein the trichloroethylene fed into the first reaction zone in step(A) together with the product stream from step (B) is added to saidproduct stream from step (B) in order to heat the trichloroethylene andcool the product stream in advance of the first reaction zone.
 11. Amethod for the manufacture of 1,1,1,2-tetrafluoroethane which comprisesthe steps of:(A) contacting a mixture of trichloroethylene and hydrogenfluoride with a fluorination catalyst under superatmospheric pressure ata temperature in the range of about 200° to 400° C. in a first reactionzone to form a product containing 1,1,1-trifluoro-2-chloroethane andhydrogen chloride together with unreacted starting materials, (B)passing the 1,1,1-trifluoro-2-chloroethane obtained in step (A) togetherwith hydrogen fluoride to a second reaction zone containing afluorination catalyst at a temperature in the range of about 280°-450°C. but higher than the temperature in step (A) to form a productcontaining 1,1,1,2-tetrafluoroethane, 1,1,1-trifluoro-2-chloroethane andhydrogen chloride, (C) treating the product of step (B) to separate1,1,1,2-tetrafluoroethane from 1,1,1-trifluoro-2-chloroethane andunreacted hydrogen fluoride, and (D) feeding the1,1,1-trifluoro-2-chloroethane obtained from step (C) and hydrogenfluoride to said first reaction zone step (A).
 12. In a method forproducing 1,1,1,2-tetrafluoroethane in two reaction stages involving (1)the reaction of trichloroethylene and hydrogen fluoride to produce1,1,1-trifluoro-2-chloroethane and (2) the reaction of1,1,1-trifluoro-2-chloroethane with hydrogen fluoride to produce1,1,1,2-tetrafluoroethane, the improvement which comprises carrying outthe reaction (2) between 1,1,1-trifluoro-2-chloroethane and hydrogenfluoride at superatmospheric pressure and at a temperature in the rangeof 280°-450° C., carrying out the reaction (1) between trichloroethyleneand hydrogen fluoride at a temperature in the range of 200°-400° C. andbelow that used in reaction (2), and recycling unconverted1,1,1-trifluoro-2-chloroethane with hydrogen fluoride for furtherreaction in the presence of trichloroethylene.
 13. In a method forproducing 1,1,1,2-tetrafluoroethane in two reaction stages involving (1)the reaction of trichloroethylene and hydrogen fluoride to produce1,1,1-trifluoro-2-chloroethane and (2) the reaction of1,1,1-trifluoro-2-chloroethane with hydrogen fluoride to produce1,1,1,2-tetrafluoroethane, the improvement which comprises carrying outthe reaction (2) between 1,1,1-trifluoro-2-chloroethane and hydrogenfluoride at a temperature in the range of 280°-450° C., carrying out thereaction (1) between trichloroethylene and hydrogen fluoride at atemperature in the range of 200°-400° C. and below that used in reaction(2), and recycling unconverted 1,1,1-trifluoro-2-chloroethene withhydrogen fluoride for further reaction in the presence oftrichloroethylene.